The present invention relates to tubular prostheses such as stents insertable into blood vessels and other body lumens, and more particularly to features in such prostheses for controlling the amount or nature of their radial expansion.
A variety of treatment and diagnostic procedures involves devices intraluminally implanted into the body of a patient. Among these devices are stents, such as disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The Wallsten devices are tubular, braided structures formed of helically wound thread elements. The stents are deployed using a delivery catheter such as disclosed in U.S. Pat. No. 5,027,377 (Burton, et al.). With the stent positioned at the intended treatment site, an outer tube of the delivery catheter is withdrawn, allowing the stent to radially expand into a substantially conforming surface contact with a blood vessel wall or other lumen-defining tissue.
An alternative stent construction features plastically deformable metallic strands, which also can be helically wound. Such stent does not require an outer tube or other feature to maintain it in a reduced-radius state during delivery. Radial expansion at the treatment site requires a dilatation balloon or other external expansion means.
Regardless of whether stents are self-expanding or plastically deformable, they characteristically have an open mesh or open frame construction, or otherwise are formed with multiple openings to facilitate radial enlargements and reductions, and to allow tissue ingrowth. Such stents typically expand axially or longitudinally as they radially contract, and conversely expand radially as they are axially shortened.
Self-expanding stents frequently are favored, not only because they do not require a balloon or other external expansion device, but also because of the radially outward residual force exerted against the surrounding tissue by a properly sized stent once deployed. A variety of constructions are available, ranging from a single coil (e.g., of a memory metal such as Nitinol) to multiple monofilaments (metalic or polymeric) arranged in sets of oppositely directed helices.
While the advantages of self-expanding stents are widely known, it would be beneficial to provide more positive control over the extent of a stent's radial expansion, once it is released from within a deployment device. Self-expanding stents characteristically have a relaxed (free of external stress) state toward which they expand when released. After delivery and release of a stent at a selected treatment site, it may appear that the relaxed state radius is less than that desired for the application. Corrective action, such as retrieving the stent and replacing it with a larger-diameter device, might be required.
Even in situations where the deployed device is considered satisfactory, it might be desired to increase or decrease, in situ, the amount of radial residual force which the device exerts against surrounding tissue. Conventional stent constructions do not afford this option.
Another consideration arises if unexpected irregularities in vasculature are encountered at the treatment site. While stents can be configured to have diameter gradients or irregular profiles, this capability does not address the desire to reconfigure a stent toward conformity with an unanticipated tissue irregularity at the treatment site.
Implanted devices are subject to vessel constriction or closure if their radially outward acting restoring force is overcome by tissue contractions.
Therefore, it is an object of the present invention to provide a body insertable prosthesis including a control feature for determining the amount of radial expansion of the prosthesis upon its release from a deployment device.
Another object is to provide a means for controllably varying the amount of radially acting restoring force in a self-expanding stent radially enlarged to a given diameter.
A further object is to provide a tubular self-expanding prosthesis radially expandable over a selected portion of its length, either to the exclusion of or to a greater degree than a remaining portion of the device length.
Yet another object is to provide a body insertable self-expanding tubular prosthesis usable in body lumens having a wider range of diameters.